Fundamentals of Python: From First Programs Through Data

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Fundamentals of Python:
From First Programs Through Data
Structures
Chapter 11
Searching, Sorting, and Complexity
Analysis
Objectives
After completing this chapter, you will be able to:
• Measure the performance of an algorithm by
obtaining running times and instruction counts with
different data sets
• Analyze an algorithm’s performance by determining
its order of complexity, using big-O notation
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Objectives (continued)
• Distinguish the common orders of complexity and
the algorithmic patterns that exhibit them
• Distinguish between the improvements obtained by
tweaking an algorithm and reducing its order of
complexity
• Write a simple linear search algorithm and a simple
sort algorithm
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Measuring the Efficiency of Algorithms
• When choosing algorithms, we often have to settle
for a space/time tradeoff
– An algorithm can be designed to gain faster run
times at the cost of using extra space (memory), or
the other way around
• Memory is now quite inexpensive for desktop and
laptop computers, but not yet for miniature devices
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Measuring the Run Time of an
Algorithm
• One way to measure the time cost of an algorithm
is to use computer’s clock to obtain actual run time
– Benchmarking or profiling
• Can use time() in time module
– Returns number of seconds that have elapsed
between current time on the computer’s clock and
January 1, 1970
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Measuring the Run Time of an
Algorithm (continued)
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Measuring the Run Time of an
Algorithm (continued)
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Measuring the Run Time of an
Algorithm (continued)
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Measuring the Run Time of an
Algorithm (continued)
• This method permits accurate predictions of the
running times of many algorithms
• Problems:
– Different hardware platforms have different
processing speeds, so the running times of an
algorithm differ from machine to machine
• Running time varies with OS and programming
language too
– It is impractical to determine the running time for
some algorithms with very large data sets
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Counting Instructions
• Another technique is to count the instructions
executed with different problem sizes
– We count the instructions in the high-level code in
which the algorithm is written, not instructions in the
executable machine language program
• Distinguish between:
– Instructions that execute the same number of times
regardless of problem size
• For now, we ignore instructions in this class
– Instructions whose execution count varies with
problem size
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Counting Instructions (continued)
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Counting Instructions (continued)
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Counting Instructions (continued)
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Counting Instructions (continued)
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Measuring the Memory Used by an
Algorithm
• A complete analysis of the resources used by an
algorithm includes the amount of memory required
• We focus on rates of potential growth
• Some algorithms require the same amount of
memory to solve any problem
• Other algorithms require more memory as the
problem size gets larger
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Complexity Analysis
• Complexity analysis entails reading the algorithm
and using pencil and paper to work out some
simple algebra
– Used to determine efficiency of algorithms
– Allows us to rate them independently of platformdependent timings or impractical instruction counts
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Orders of Complexity
• Consider the two counting loops discussed earlier:
• When we say “work,” we usually mean the number
of iterations of the most deeply nested loop
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Orders of Complexity (continued)
• The performances of these algorithms differ by
what we call an order of complexity
– The first algorithm is linear
– The second algorithm is quadratic
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Orders of Complexity (continued)
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Big-O Notation
• The amount of work in an algorithm typically is the
sum of several terms in a polynomial
– We focus on one term as dominant
• As n becomes large, the dominant term becomes
so large that the amount of work represented by
the other terms can be ignored
– Asymptotic analysis
• Big-O notation: used to express the efficiency or
computational complexity of an algorithm
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The Role of the Constant of
Proportionality
• The constant of proportionality involves the
terms and coefficients that are usually ignored
during big-O analysis
– However, when these items are large, they may
have an impact on the algorithm, particularly for
small and medium-sized data sets
• The amount of abstract work performed by the
following algorithm is 3n + 1
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Search Algorithms
• We now present several algorithms that can be
used for searching and sorting lists
– We first discuss the design of an algorithm,
– We then show its implementation as a Python
function, and,
– Finally, we provide an analysis of the algorithm’s
computational complexity
• To keep things simple, each function processes a
list of integers
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Search for a Minimum
• Python’s min function returns the minimum or
smallest item in a list
• Alternative version:
n – 1 comparisons for a list of size n
• O(n)
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Linear Search of a List
• Python’s in operator is implemented as a method
named __contains__ in the list class
– Uses a sequential search or a linear search
• Python code for a linear search function:
– Analysis is different from previous one
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Best-Case, Worst-Case, and AverageCase Performance
• Analysis of a linear search considers three cases:
– In the worst case, the target item is at the end of the
list or not in the list at all
• O(n)
– In the best case, the algorithm finds the target at the
first position, after making one iteration
• O(1)
– Average case: add number of iterations required to
find target at each possible position; divide sum by n
• O(n)
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Binary Search of a List
• A linear search is necessary for data that are not
arranged in any particular order
• When searching sorted data, use a binary search
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Binary Search of a List (continued)
– More efficient than linear search
• Additional cost has to do with keeping list in order
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Comparing Data Items and the cmp
Function
• To allow algorithms to use comparison operators
with a new class of objects, define __cmp__
– Header: def __cmp__(self, other):
– Should return:
• 0 when the two objects are equal
• A number less than 0 if self < other
• A number greater than 0 if self > other
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Comparing Data Items and the cmp
Function (continued)
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Comparing Data Items and the cmp
Function (continued)
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Sort Algorithms
• The sort functions that we develop here operate on
a list of integers and uses a swap function to
exchange the positions of two items in the list
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Selection Sort
• Perhaps the simplest strategy is to search the
entire list for the position of the smallest item
– If that position does not equal the first position, the
algorithm swaps the items at those positions
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Selection Sort (continued)
• Selection sort is O(n2) in all cases
• For large data sets, the cost of swapping items
might also be significant
– This additional cost is linear in worst/average cases
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Bubble Sort
• Starts at beginning of list and compares pairs of
data items as it moves down to the end
– When items in pair are out of order, swap them
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Bubble Sort (continued)
• Bubble sort is O(n2)
• Will not perform any swaps if list is already sorted
• Worst-case behavior for exchanges is greater than
linear
• Can be improved, but average case is still O(n2)
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Insertion Sort
• Worst-case behavior of insertion sort is O(n2)
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Insertion Sort (continued)
• The more items in the list that are in order, the
better insertion sort gets until, in the best case of a
sorted list, the sort’s behavior is linear
• In the average case, insertion sort is still quadratic
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Best-Case, Worst-Case, and AverageCase Performance Revisited
• Thorough analysis of an algorithm’s complexity
divides its behavior into three types of cases:
– Best case
– Worst case
– Average case
• There are algorithms whose best-case and
average-case performances are similar, but whose
performance can degrade to a worst case
• When choosing/developing an algorithm, it is
important to be aware of these distinctions
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An Exponential Algorithm: Recursive
Fibonacci
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An Exponential Algorithm: Recursive
Fibonacci (continued)
• Exponential algorithms are generally impractical to
run with any but very small problem sizes
• Recursive functions that are called repeatedly with
same arguments can be made more efficient by
technique called memoization
– Program maintains a table of the values for each
argument used with the function
– Before the function recursively computes a value for
a given argument, it checks the table to see if that
argument already has a value
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Converting Fibonacci to a Linear
Algorithm
• Pseudocode:
Set sum to 1
Set first to 1
Set second to 1
Set count to 3
While count <= N
Set sum to first + second
Set first to second
Set second to sum
Increment count
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Converting Fibonacci to a Linear
Algorithm (continued)
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Case Study: An Algorithm Profiler
• Profiling: measuring an algorithm’s performance,
by counting instructions and/or timing execution
• Request:
– Write a program to allow profiling of sort algorithms
• Analysis:
– Configure a sort algorithm to be profiled as follows:
• Define sort function to receive a Profiler as a
2nd parameter
• In algorithm’s code, run comparison() and
exchange() with the Profiler object where
relevant, to count comparisons and exchanges
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Case Study: An Algorithm Profiler
(continued)
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Case Study: An Algorithm Profiler
(continued)
• Design:
– Two modules:
• profiler—defines the Profiler class
• algorithms—defines the sort functions, as
configured for profiling
• Implementation (Coding):
– Next slide shows a partial implementation of the
algorithms module
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Case Study: An Algorithm Profiler
(continued)
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Summary
• Different algorithms can be ranked according to
time and memory resources that they require
• Running time of an algorithm can be measured
empirically using computer’s clock
• Counting instructions provides measurement of
amount of work that algorithm does
• Rate of growth of algorithm’s work can be
expressed as a function of the size of its problem
instances
• Big-O notation is a common way of expressing an
algorithm’s runtime behavior
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Summary (continued)
• Common expressions of run-time behavior are
O(log2n), O(n), O(n2), and O(kn)
• An algorithm can have different best-case, worstcase, and average-case behaviors
• In general, it is better to try to reduce the order of
an algorithm’s complexity than it is to try to
enhance performance by tweaking the code
• Binary search is faster than linear search (but data
must be ordered)
• Exponential algorithms are impractical to run with
large problem sizes
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